Conventional part-time four-wheel drive vehicles are typically equipped with a drivetrain to transmit rotary power from the power train to a front pair of drive wheels and a rear pair of drive wheels. The front axle assembly is connected to a front driveshaft and the rear axle assembly is connected to a rear driveshaft. In four-wheel drive mode, the front driveshaft and the rear driveshaft are locked together in order to rotate at the same rate. The locking mechanism is in the transfer case and includes a chain connecting the front driveshaft to the rear driveshaft via a drive sprocket and a driven sprocket.
In many four-wheel drive vehicles, the front brake system is larger than the rear brake system. In this case, when the brakes are applied, the front wheels stop rotating before the rear wheels. When brakes are applied to a vehicle being driven in four-wheel drive mode, the chain in the transfer case sometimes jumps over a tooth on one or both of the drive sprocket and driven sprocket.
In order to prevent chain jump, the chain is modified in various ways, which increase cost and weight with a resultant reduction in fuel efficiency. The chain modifications used to prevent chain jump include increasing the chain width, pitch and strength.